Electrostatic chucks have been used to hold wafers by electrostatic attraction force in the etching step to form fine patterns on the semiconductor wafers (hereinafter referred to as a wafer) and the film forming step to form thin films, in the semiconductor manufacturing process.
The electrostatic chuck comprises a dielectric layer, a top surface of which is used as a mounting surface whereon a wafer is attracted and mounted, and an electrostatic attraction electrode provided on the bottom surface of the dielectric layer, wherein a voltage is applied between the wafer and the electrostatic attraction electrode so as to generate electrostatic attraction force and thereby hold the wafer on the mounting surface.
When manufacturing semiconductors today, it is required to etch the wafer at a constant etching rate throughout the wafer surface and form the thin film with uniform thickness. This makes it necessary to keep the wafer at a constant temperature and minimize the temperature difference across the wafer surface. In recent years, with the rapid growth of the semiconductor industry, demand has been increasing for processing a larger number of wafers per unit time, and it is required to shorten the time taken to heat the wafer to the saturation temperature.
Japanese Unexamined Patent Publication No. 9-172055 proposes an electrostatic chuck having a plurality of tiny protrusions, each tapering off from the base toward the tip thereof, provided on the mounting surface so that a wafer is held via point contacts at the tips of the protrusions, in order to put the wafer into contact with a gas heating the wafer through larger area and thereby heat the wafer uniformly.
Japanese Unexamined Patent Publication No. 2002-222851 describes that by decreasing the contact area between the wafer and the electrostatic chuck surface and decreasing the height of the protrusions, it is possible to attract the wafer by the force exerted not only by the contact portion but also by other portions, thereby resulting in that the wafer can be held by a larger force with a smaller contact area and the wafer can be heated uniformly.
With the electrostatic chucks described in Japanese Unexamined Patent Publication No. 9-172055 and Japanese Unexamined Patent Publication No. 2002-222851, however, the contact area between the wafer and the electrostatic chuck is too small and the wafer can be lifted of the electrostatic chuck by the pressure of a gas which is caused to flow through a gas flow passage of the electrostatic chuck in the case of an electrostatic chuck that utilizes weak Coulomb force, although it does not cause a problem in the case of electrostatic chuck that utilizes Johnson Rahbeck effect which exerts larger attraction force. If the gas pressure is decreased so that the wafer would not be lifted off the electrostatic chuck, it takes a longer time to heat the wafer to the saturation temperature, thus decreasing the number of wafers which can be processed per unit time.
In order to increase the contact area between the wafer and the mounting surface so that the wafer would not be lifted off the electrostatic chuck, Japanese Unexamined Patent Publication No. 2002-170868 describes an electrostatic chuck which is capable of heating a wafer to the saturation temperature in a shorter time and achieve uniform temperature distribution at the saturation temperature, with such a constitution as the electrostatic chuck has a plurality of gas grooves disposed to run in the radial directions at equal intervals, a plurality of annular grooves disposed in concentric circles in communication with the radial grooves, and at least one separating groove extending in the radial direction so as to divide each mounting surface region, surrounded by two annular grooves and two radial grooves, located outward from the annular groove that is nearest to the center, into two parts, wherein the mounting surface region surrounded by two adjacent annular grooves and two adjacent radial grooves, mounting surface region surrounded by two adjacent annular grooves, one radial groove and one separation groove and mounting surface region surrounded by two adjacent annular grooves and two separation grooves have substantially the same area.
Japanese Unexamined Patent Publication No. 7-153825 discloses an electrostatic chuck having protrusions of circular shape disposed in a grating pattern on the wafer mounting surface.